[mattdotc]

As others have stated, that's not necessarily going to do the trick, either. I'll make a huge leap here and guess that you're a primarily laptop user. Don't forget about desktop users.

I use an external amplifier connected through a front-panel IO port. Both the front panel and my amplifier have volume knobs. I don't use the system try to control my levels. I know I'm an odd case, but surely there many people out there with basic integrated speaker setups with volume knobs.

The simplest solution would be to visually indicate to your user as early as possible that your page uses audio. If it is designed to be particularly loud or quiet, you can mention that, too.